Feature/remove node mutations

[dxinteractive]

• Add validation example Demo: add multi-field form validation example #15 - as yet it is not genericised, but will be for the plugin Plugin: add validation plugin #18
• Added descriptions Demo: add descriptions #28

Nodes

• Stop mutating child nodes Perf: stop relying on mutations within Nodes #23
• Child nodes are always created upon attempted access, regardless of the existence of the chilren in the underlying data
• Child nodes are not batch created upon parent access anymore, they are only created when the children themselves have attempted access
• Changed expected behaviour - now if an object, map or basic type is removed, its node data and therefore information about its children is retained, not removed, so the user doesnt have to think about the depth of the change they are making (which is trickier to conceptualise with immer in the picture too). Arrays node data is removed as usual, because the dendriform-immer-patch-optimiser attempts to deal with child identification on arrays
• Fixed bug where new nodes were needlessly created because immer sends an "add" instead of a "replace" referring to a path that already has a value on it. Unsure whether this is an immer issue or a dendriform one. This case is easily bandaided for now.